1. Field of the Invention
The present invention relates to an electro-chemical capacitor.
2. Description of the Related Art
Electrochemical capacitors are a device for storing energy in an interface between an electrode as an electron conductor and an electrolyte as an ion conductor, and are classified into electric double-layer capacitors and pseudo capacitors (redox capacitors).
An electric double-layer capacitor comprises electrode elements as an anode and a cathode that are disposed in confronting relationship to each other with a separator interposed therebetween. Both the anode and the cathode comprise polarized electrodes made of a formed body of fibers or particles of activated carbon and a coated film of particles of activated carbon, the electrode elements being impregnated with an electrolyte. The electric double-layer capacitor stores electric charges in electric double layers produced in the interfaces between the polarized electrodes and the electrolyte.
A pseudo capacitor comprises electrode elements as an anode and a cathode that are disposed in confronting relationship to each other with a separator interposed therebetween. One of the anode and the cathode comprises a polarized electrode made of a formed body of fibers or particles of activated carbon and a coated film of particles of activated carbon, and the other of a non-polarized electrode made of a metal oxide or an electrically conductive polymer, the electrode elements being impregnated with an electrolyte. When a potential difference across the interface between the non-polarized electrode and the electrolyte is changed, electric charges move in the non-polarized electrode. The metal oxide may comprise ruthenium oxide, iridium oxide, nickel oxide, lead oxide, or the like, and the electrically conductive polymer may comprise polypyr-role, polythiophene, or the like. The pseudo capacitor of the above structure utilizes the phenomenon in which electric charges are stored in an electric double layer produced in the interface between the polarized electrode and the electrolyte, and also utilizes a pseudo capacitance due to the movement of electric charges caused by an oxidation and reduction in the vicinity of the interface between the non-polarized electrode and the electrolyte (see B. E. Conway, J. Electrochem. Soc., 138, 1539 (1991)).
The electrolytes used in the electrochemical capacitors are roughly classified into liquid electrolytes (electrolytic liquids) and solid electrolytes from their, state, and the electrolytic liquids are also classified aqueous and nonaqueous ones from a kind of solvent. The nonaqueous electrolytic liquids comprise electrolytic solutions prepared by dissolving quaternary ammonium salt, quaternary phosphonium salt, etc. into an organic solvent of propylene carbonate, etc. The solid electrolytes include a polyethylene oxidexe2x80x94alkali metal salt complex, RbAg4I5, etc. (Makoto Ue, xe2x80x9cElectrochemistryxe2x80x9d, 66, 904 (1998)).
If an electrochemical capacitor uses a nonaqueous electrolytic liquid, then it has an increased withstand voltage and also has a higher energy density than if it uses an aqueous electrolytic liquid. Electrochemical capacitors using nonaqueous electrolytic liquids find use as backup power supplies for electronic devices that need to be smaller in size and lower in profile, and are also suitable for use in power applications such as electric vehicles, hybrid vehicles, and power storage devices which have been drawing attention in recent years.
When the electrochemical capacitor utilizes the phenomenon in which electric charges are stored in an electric double layer produced in the interface between the polarized electrode and the electrolyte, the energy W accumulated in the electric double layer at the time the electro-chemical capacitor is discharged at a constant current I from a voltage Vi to a voltage Vf is expressed by the following equation (1):                                                         W              =                                                1                  /                  2                                ·                C                ·                                  (                                                            V                      i                      2                                        -                                          V                      f                      2                                                        )                                                                                                        =                                                1                  /                  2                                ·                C                ·                                  [                                                                                    (                                                                              V                            0                                                    -                          IR                                                )                                            2                                        -                                          V                      f                      2                                                        ]                                                                                        (        1        )            
Therefore, in order to increase the energy density of the electrochemical capacitor which utilizes the phenomenon in which electric charges are stored in the electric double layer, it is necessary to increase the capacitance C (F) or the open-circuit voltage V0 (V) or to reduce the internal resistance R (xcexa9). The capacitance C increases in proportion to the effective surface area of contact between the polarized electrode and the electrolyte, and is determined by a withstand voltage that is determined by the reactivity between the polarized electrode and the electrolyte. The internal resistance R includes the electric resistance of the electrode itself, and also a diffusion resistance for ions to move in pores of the electrode and a diffusion resistance for ions to move in the electrolyte. The diffusion resistance for ions to move in the electrolyte is in inverse proportion to the electric conductivity of the electrolyte. Consequently, the electrolyte is generally desired to have a high electric conductivity.
Japanese laid-open patent publication No. 63-173312 discloses, as the above electrolyte, a nonaqueous electrolytic liquid prepared by dissolving an asymmetric quaternary ammonium salt as an electrolytic salt into a nonaqueous solvent. The electrolyte is used in an electric double-layer capacitor which uses a polarized electrode of activated carbon as each of an anode and a cathode. The above publication states that an electric double-layer capacitor having a low internal resistance, a low capacitance degradation ratio under high temperature conditions, and excellent long-term reliability is produced by using an electrolytic liquid which employs the above electrolytic salt.
However, in view of stricter performance demands for electrochemical capacitors in recent years, there has been desired the development of electrochemical capacitors of lower internal resistance, greater capacitance, and higher energy density.
It is therefore an object of the present invention to provide an electrochemical capacitor of high energy density.
To achieve the above object, an electrochemical capacitor comprises electrode elements as an anode and a cathode that are disposed in confronting relationship to each other with a separator interposed therebetween, and a nonaqueous electrolytic liquid impregnated in the electrode elements, the nonaqueous electrolytic liquid being prepared by dissolving an electrolytic salt Q+Axe2x88x92 including cations Q+ whose van der Waals volume ranges from 0.10 to 0.125 nm3 into a nonaqueous solvent, the cathode comprising a polarized electrode made of activated carbon having pore diameters whose mode is at most 1.5 nm.
Since the van der Waals volume of the cations Q+ ranges from 0.10 to 0.125 nm3, the cations Q+ can be diffused relatively freely in the pores of the activated carbon of the polarized electrode. If the van der Waals volume of the cations Q+ were smaller than 0.10 nm3, then they strongly interact with the molecules of the nonaqueous solvent, producing large solvation ions which prevent the cations Q+ from being diffused in the pores of the activated carbon. If the van der Waals volume of the cations Q+ were greater than 0.125 nm3, then the cations Q+ are prevented from being diffused in the pores of the activated carbon due to the size of the cations Q+ themselves.
The van der Waals volume of the cations Q+ can be calculated from a model in which spherical atoms of the cations Q+ are bonded and superposed at a given bonding distance and angle (see M. Ue, J. Electrochem. Soc., 141, 3336 (1994)). For the radii of the spherical atoms, numerical values proposed by A. Bondi, J. Phys. Chem., 68, 441 (1964) (see xe2x80x9cChemical Manual, Basics IIxe2x80x9d, edited by Chemical Society of Japan, H: 0.120 nm, C: 0.170 nm, N: 0.155 nm, P: 0.180 nm, As: 0.185 nm, Sb: 0.210 nm) are employed. Values measured by an X-ray diffraction analysis or a neutron diffraction analysis of an ion crystal including cations Q+ are used for conformations. Alternatively, conformations obtained by xe2x80x9cMM2 calculationsxe2x80x9d known as a molecular force field calculating program may be used. According to the MM2 calculations, the value of an obtained volume slightly differs depending on an interstitial distance for integrating the numerical value of the volume. According to the present invention, the interstitial distance is set to a constant value of 0.075 nm.
In the electrochemical capacitor according to the present invention, since the cations Q+ are defined, at least the cathode of the electrode elements is constructed as the polarized electrode. With the cathode being constructed as the polarized electrode, when an electric double layer is produced in the interface between the cathode and the electrolytic liquid including the cations Q+ that are diffused relatively freely in the pores of the activated carbon, the cations Q+ are collected toward the electrolytic liquid, and the polarized electrode is charged by anions, making the electrochemical capacitor advantageous.
The polarized electrode for use as the cathode is made of activated carbon having pore diameters whose mode is at most 1.5 nm. Therefore, there are available many pores in which cations Q+ whose van der Waals volume is in the above range can be diffused relatively freely, resulting in an increased capacitance. If the mode of the pore diameters of the activated carbon were greater than 1.5 nm, there are available fewer pores whose diameter is at most 1.5 nm, resulting in a reduction in the capacitance per unit volume of the activated carbon.
The mode of the pore diameters of the activated carbon can be determined as follows: First, a powder of activated carbon is dispersed in a suitable solvent, and the solution is dropped onto a mesh applied to a microgrid for use with a transmission electron microscope. The solution on the mesh is then dried in the atmosphere, thus producing an observation sample. The observation sample is then imaged at an acceleration voltage of 200 kV by a transmission electron microscope (CM200 manufactured by Philips, with an objective lens having a spherical aberration coefficient Cs=1.2 mm). At this time, the aperture of the objective lens is adjusted to exclude diffraction rays having a larger diffraction angle than the 002 face of graphite and focus the image for thereby cutting off the information that is more specific than the distance of the 002 face of graphite. Then, the produced negative image is read by a film scanner to produce digital image data having an image size of 512xc3x97512 pixels and 256 gradations. The produced digital image data is subjected to two-dimensional Fourier transform, and the produced power spectrum is circularly integrated to produce a one-dimensional power spectrum. The one-dimensional power spectrum represents an extracted periodicity of irregularities of the activated carbon, and reflects a distribution of pore diameters of the activated carbon. The value of the center of the Gaussian distribution with which the one-dimensional power spectrum has recurred is used as the mode of pore diameters. The mode of pore diameters thus determined according to the above process will hereinafter be referred to as the mode of pore diameters determined by an image analysis of transmission electron microscope images.
With the above structure, the combination of a nonaqueous electrolytic liquid and a polarized electrode can be optimized to provide an electrochemical capacitor having a low internal resistance, a large capacitance, and a high energy density.
In the electrochemical capacitor, the cations Q+ may comprise organic onium ions represented by (CH3)3(C2H5)X+ (X=N, P, As) or (CH3)4X+ (X=P, As, Sb).
The anions Axe2x88x92 which cooperate with the cations Q+ in forming the electrolytic salt Q+Axe2x88x92 should preferably comprise fluorine complex ions represented by MFnxe2x88x92 (M=B, n=4 or M=P, As, Sb, n=6) as a high withstand voltage and a high electric conductivity are achieved when the electrolytic salt Q+Axe2x88x92 is formed.
The electrolytic salt Q+Axe2x88x92 for use in the electrochemical capacitor may be any salt that is obtained by the combination of the cations Q+ and the anions A+. However, (CH3)3(C2H5)NBF4 is particularly preferable because its toxicity is low and its industrial production is easy.
The nonaqueous solvent should preferably comprise propylene carbonate for its high withstand voltage and high electric conductivity, and a wide temperature range in which it can be used.
The nonaqueous electrolytic liquid should preferably be prepared by dissolving (CH3)3(C2H5)NBF4 as the electrolytic salt Q+Axe2x88x92 into the propylene carbonate because its toxicity is low and its industrial production is easy, its withstand voltage and electric conductivity are high, and it can be used in a wide temperature range. The nonaqueous electrolytic liquid can be used in a concentration ranging from 0.3 to 3.0 moles/liter. If the concentration of the nonaqueous electrolytic liquid were lower than 0.3 mole/liter, then the electric conductivity of the nonaqueous electrolytic liquid would be lowered, resulting in an increase in the internal resistance of the electro-chemical capacitor. If the concentration of the nonaqueous electrolytic liquid were higher than 3.0 moles/liter, then salt would be precipitated at low temperatures. These drawbacks can reliably be eliminated if the nonaqueous electrolytic liquid is used in a concentration ranging from 0.5 to 2.0 moles/liter.
In the electrochemical capacitor according to the present invention, at least the anode may comprise the polarized electrode, as described above. If more importance is attached to energy capacity, then the anode may comprise a non-polarized electrode of metal oxide, conductive polymer, or the like. If more importance is attached to service life and reliability, then the anode may comprise a polarized electrode. If more importance is attached to the manufacturing cost, then the anode should preferably comprise a polarized electrode of activated carbon.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.